Usually, for a high temperature heat resistant part of a boiler, a heat exchanger, or the like, an internally ribbed steel tube (rifled tube) with spiral ribs (protrusions) formed on the internal surface of the steel tube is used to improve a power generation efficiency. Since the internal surface of the internally ribbed steel tube has a larger surface area by the ribs formed on the internal surface, a contact area between water vapor passing through the inside of heated tube and the internal surface of the tube increases, while allowing turbulence to occur in a fluid containing water vapor, thereby enabling a heat exchange efficiency to be enhanced. With a recent tendency of increased capacity and higher temperature/higher pressure of the boiler, the demand for the internally ribbed steel tube has increased rapidly.
To produce the internally ribbed steel tube, a seamless steel tube or an electric resistance welded steel tube is used as a blank tube, the blank tube is sufficiently softened as necessary, and then in a cold working process a drawing die and a plug, which has spiral grooves on its outer peripheral surface for forming ribs for the tube, are used to draw the tube.
FIG. 1 is an explanatory view for schematically illustrating a production method of an internally ribbed steel tube by cold drawing. When a blank tube 3 is cold drawn, a plug 1 is inserted into the blank tube 3 in a concentric manner relative to a die 2 and the blank tube 3, and the blank tube 3 is drawn in the direction indicated by a hollow arrow while allowing the plug 1 to be rotated. The external surface of blank tube 3 is reduced by the die 2. The internal surface of the blank tube 3 is pressed into and processed along spiral grooves 1a made on the outer peripheral surface of the plug 1 so that spiral ribs 3a are formed on the inner peripheral surface of the drawn blank tube 3.
The plug 1 thus used can be rotated freely, and is held by a mandrel 4. The plug shape greatly affects qualities such as rib height and rib shape (especially, rib corner part and lead angle) of the internally ribbed steel tube, and the seizure defective occurs between the blank tube and the plug depending on drawing conditions.
Therefore, regarding the production of internally ribbed steel tube, various proposals have conventionally been made on the configuration and shape of the plug. For example, Japanese Patent Application Publication No. 2001-179327 proposes a plug in which in a spiral groove thereof, the radius of curvature for each of corner portions where both groove side walls intersect a groove bottom surface is kept constant all the way from the front end of the plug to the rear end thereof, and the diameter of the plug is decreased at a fixed gradient from the front end of the plug toward the rear end thereof.
Also, Japanese Patent Application Publication No. 2006-272392 has disclosed a drawing tool for drawing the internally ribbed steel tube, in which edges of each spiral groove ridge are rounded or chamfered linearly to reduce the area of contact between the top land part of groove ridge and the blank tube, thereby reducing frictional resistance between the groove ridge top part and the blank tube.